Although many people assume grapefruit is a harmless fruit, it actually carries some serious safety concerns that should be considered, particularly for people who take medications regularly.
Grapefruit interacts with nearly half of all oral medications. These include medications used to manage epilepsy, high cholesterol, HIV, heart arrhythmias, high blood pressure, organ transplants, anxiety, erectile dysfunction, pain, and more.[1][2] For a list of medications that could be affected, see here. This interaction can occur with as little as 6 ounces (180 mL) of grapefruit juice or about 180 grams of fresh grapefruit and persists for at least 24 hours after consumption.[3]
Regarding side effects, grapefruit is generally considered well tolerated when consumed in appropriate amounts. Most studies have failed to report on the adverse effects of grapefruit, apart from one study that reported both constipation and diarrhea during treatment with grapefruit capsules.[4]
Consuming grapefruit in large amounts (≥1 liter of juice at once) seems to prolong the QTc interval (a measurement of the heart's electrical activity). This could increase the risk of torsades de pointes, a potentially fatal heart arrhythmia. This effect has been observed in multiple randomized controlled trials and appears greater in women and in people with pre-existing disease affecting the heart rhythm.[5][6][7] The flavanone naringenin may be the culprit. In one study, QTc prolongation was observed at doses of about 1440 µmol naringenin (equivalent to roughly 390 mg). While naringenin content in grapefruit varies, this could correspond to about 4.5 whole grapefruits.[8]